As ferromagnetic fine powder for magnetic recording medium, there are most generally used magnetic powders of needle crystals of .gamma.-Fe.sub.2 O.sub.3, cobalt coated .gamma.-Fe.sub.2 O.sub.3, iron-type metals, Cr.sub.2 O.sub.3 and the like. These magnetic powders have a needle-like particle shape and hence are not sufficient in packing density. Cobalt coated .gamma.-Fe.sub.2 O.sub.3 permits enhancement of coercive force but is not satisfactory in the thermal stability of coercive force, etc. Further in the case of the iron-type metals, a high saturation magnetization can be attained, but a problem of so-called low corrosion resistance is unavoidable and there are not a few problems, for example, easy deterioration of the stability with the lapse of time of magnetic characteristics. In addition, in the case of the above-mentioned needle crystals magnetic powders, magnetic recording and processing is conducted by a so-called longitudinal recording method which comprises coating on a base film a coating liquid prepared by dispersing the magnetic powder in a binder resin, thereby forming a magnetic recording medium, orienting the needle crystals in the direction of inside the surface of the medium, and utilizing the residual magnetization in said direction. It is well known that in such a recording method, enhancement of the recording density by use of the aforesaid needle crystal magnetic powder is very difficult because an attempt to increase the density of magnetic recording results in an increase of the demagnetizing field in the medium and tends to deteriorate the playback characteristics particularly in the short wavelength region.
On the other hand, in consideration of the above problems in the case where the aforesaid needle crystals magnetic powders are used, attempts have been made in recent years to enhance the recording density by applying a perpendicular magnetic recording method which comprises forming a coated-form magnetic recording medium by use of powder consisting of platelet like hexagonal ferrite crystal particles, e.g., barium ferrite crystal particles, and magnetizing the recording medium in a direction perpendicular to the surface of the recording medium. Various barium ferrite magnetic powders used in the above recording method have been proposed. Many of them consist of magnetoplumbite type barium ferrite crystal particles composed of BaO.multidot.nFe.sub.2 O.sub.3 (n=5 or 6) in which a portion of the Fe atoms have been usually replaced by a very small amount of various foreign metallic elements such as Co and Ti for controlling the coercive force. Such barium ferrite magnetic powders consist of platelet like particles and hence are excellent in surface smoothness and packing density in magnetic recording medium. However, only those having a saturation magnetization of at most about 60 emu/g have been obtained, and improvement of the saturation magnetization is eagerly desired for increasing the output of magnetic recording medium.
It is known with respect to the saturation magnetization of barium ferrite crystals, that crystals of W-type crystal phase (Ba.multidot.Me.sup.2+ Fe.sub.16 O.sub.27) have a higher saturation magnetization than do crystals of the above-mentioned magnetoplumbite type crystal phase. It has been also proposed to utilize the crystals of W-type crystal phase as a high-saturation-magnetization ferrite-based magnetic recording material. The W-type barium ferrite magnetic powders according to this proposal are obtained usually by preparing a blend of metallic elements having a predetermined composition, and calcining the blend at a high temperature of about 1100.degree.-1350.degree. C. The barium ferrite magnetic powders thus obtained have a high saturation magnetization, but they tend to undergo interparticle sintering or particle coarsening, so that deterioration of their dispersibility and orientation is unavoidable. Thus, they are not satisfactory as high-packing-density magnetic recording materials and involve not a few problems which should be improved. However with the tendency toward enhancement of the recording density, S/N ratio and output of magnetic recording medium, there has recently been more and more desired ferrite-based ferromagnetic powder which has a still finer particle size of 0.3 .mu.m or less, preferably 0.2 .mu.m or less and a high saturation magnetization.